Hydraulic controls are conventionally provided with a pilot stage filter to protect against contamination clogging one of the orifices in the driver stage of the hydraulic control and thereby causing the spool to go hard-over. However, such filters are not completely reliable, resulting in occasional hard-over failure.
As will be obvious, a hard-over failure can result in significant property damage and/or personal injury in many applications. Thus, if the hydraulic fluid supplied to the driver stage of a hydraulic control becomes highly contaminated, the desired mode of failure due to contamination is for the spool to move to the center, or neutral, position, thereby shutting off all flow from the valve to the load ports. This action is termed "fail-to-neutral" movement herein.